1. Moloney murine sarcoma virus (MSV) specific unintegrated circular DNA was molecularly cloned in bacteriophage lambda. During propagation of cloned recombinant DNA in Escherichia coli, specific deletions of viral information were observed. The heteroduplex mapping and restriction enzyme analysis showed that the deletions in virus specific sequences were primarily due to tandemly arranged redundant sequences. 2. The biochemical and immunological analysis of proteins coded by McDonough (SM), Gardner-Arnstein (GA) and Synder-Theilen (ST) isolates of feline sarcoma virus revealed that GA- and ST- isolates possess common tryptic fingerprints and shared antigenic determinants. These results indicate that these viruses may have been generated by acquisition of similar subsets of cellular sequences. 3. The major translational product coded by ST-FeSV has been shown to be associated with protein kinase activity which catalyzes phosphorylation of tyrosine residues present in heavy chain of immunoglobulin and itself. The resemblance of this unusual protein kinase activity of FeSV-coded protein with analogous proteins of other mammalian transforming viruses suggests that these viruses may have similar mechanism for transformation.